Effect
Single Out
Single Out is a passive that grants you a significantly higher critical hit chance against isolated enemies standing far from their allies. It's tailored to single-target situations and shines especially when fighting lone bosses or elites operating away from the pack, where you reliably reap the crit bonus.
The slot is most worthwhile in boss and single-target builds where you engage your main target separately anyway and convert the extra crit chance straight into more damage. In dense pack builds, where enemies constantly cluster together, the effect rarely triggers and the slot offers little. So you pick this passive when your build is built around cutting down isolated, dangerous targets and you want to grab every possible crit advantage.
Reading the condition closely
You gain 25% Critical Hit Chance against enemies who are more than 20 yards away from any other enemy. What counts is the target's distance to other enemies, not your own distance to the target. Whether you attack point-blank or from far away makes no difference: a lone Rift Guardian in an empty room meets the condition, while an enemy in the middle of its group does not.
When the bonus kicks in during a fight
Enemies that appear in groups only become easy prey once the other enemies around them have fallen or moved far enough away. In practice the passive stays quiet while you clear a pack, but as soon as only a few stragglers remain, it raises your critical hit chance against exactly those. Against a boss, on the other hand, it often works from start to finish.
Also watch for skills that pull enemies together or bunch them up: they are handy for area damage but prevent the condition from being met.
Available late in the game
Single Out only unlocks at level 68, so it barely matters while leveling. In the endgame it is an option for setups whose strength lies in single-target damage; against packs, passives without a distance condition usually pay off more.